Adsorption Isotherm of Aqueous Oxalic Acid on Activated Charcoal Lab Report

Adsorption definition: the adhesion in an extremely thin layer of molecules (as gases, solutes, or liquids) to the surfaces of solid bodies or liquids with which they are in contact. The binding to the surface is usually weak and reversible (which means that attached molecules can get released from the surface).

A number of extensive applications would be found in the industry such as gas masks, clarification of sugar, paint Industry, catalysis, chromatographic analysis, hard water softening, wastewater treatment, etc.

The most common industrial adsorbents are activated carbon, silica gel, and alumina because they present an enormous surface area per unit weight. Activated carbon is produced by roasting organic material to decompose it into granules of carbon - coconut shell wood and bone are common sources. Silica Gel is a matrix of hydrated silicon dioxide (SiO₂) and Alumina is mined or precipitated Aluminum oxide (Al₂O₃) and hydroxide.

They are two principles models of adsorption of molecules on surfaces:

1. Physical adsorption (physisorption)
2. Chemical adsorption (Chemisorption)

The basis of distinction is the nature of the bonding between the molecule and the surface.

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Difference of physical adsorption and chemisorption

Physical Adsorption

Physical adsorption resembles the condensation of gases to liquids and depends on the physical, or van der Waals, the force of attraction between the solid adsorbent and the adsorbate molecules. There is no chemical specificity in physical adsorption, any gas tends to be adsorbed on any solid if the temperature is sufficiently low or the pressure of the gas sufficiently high.

Chemisorption

Chemisorption is a kind of adsorption, which involves a chemical reaction between the solid exposed surface and the adsorbate. During the chemical reaction, a distinct chemical specie is attached to the adsorbent surface, which causes the bond to be created.

Chemisorption is used in the development, monitoring, and measuring of corrosion inhibitors and for measuring surface active sites on a substrate.

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Modeling with Freundlich isotherm and the Langmuir isotherm

In this experiment, the adsorption of oxalic acid on charcoal is studied by adsorption modeling with both the Freundlich isotherm and the Langmuir isotherm. This is an example of the physical adsorption principle, where dipole forces and Van der Waals forces (Van der Waals forces are much weaker than dipole forces) are the predominant sources of attraction and the heat of adsorption is typically less than 50 kJ/mol.

Factors of deciding absorbed amount to adsorbate (oxalic acid) to adsorbent (charcoal)

The amount of oxalic acid (adsorbate) adsorbed per gram of charcoal (adsorbent) will depend on the surface area of the charcoal, the temperature of the solution, and the adsorbate concentration in the solution.

The adsorption will be followed by titrating the Oxalic acid not absorbed by the charcoal and then determining the amount adsorbed by the difference in oxalic acid concentration. Isotherms (plots of moles of adsorbate adsorbed per gram of adsorbent versus solution concentration) will be constructed, and then compared with two models.

1. Freundlich isotherm
2. Langmuir isotherm

Temperature effects on adsorption are considered zero, as the measurements are usually done at a constant temperature. Graphs, obtained from that data are called isotherms.

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Adsorption isotherm and effect of temperature and concentration of adsorbate on adsorption amount

• All adsorption processes are exothermic, thus the amount of substance at any surface decreases with the rising of temperature.
• At constant temperature, the amount adsorbed increases with the concentration of adsorbate (in the gas phase/in solution).
• adsorption isotherm: The relationship between the amount (mass) adsorbed pre-unit mass of solid (Se) and the equilibrium concentration in solution (Ce) is known as the adsorption isotherm. Only at a very low concentration is proportional to Ce.

The earliest and simplest theoretical model on adsorption is that of Langmuir (1918) for gas adsorbed on solids.

Langmuir isotherm for a solution- solid interface

• Q_e is the mass of adsorbate absorbed on unit mass of absorbent (g/kg)
• C_e is the equilibrium adsorbate concentration in the solution (g/L)
• K₁ is Langmuir affinity coefficient (L/g)
• Q_max is the Langmuir adsorption capacity of adsorbate on unit mass of adsorbent (g/kg)

Simplified form of Langmuir adsorption isotherm

Freundlich isotherm

• Q_e is the mass of adsorbate absorbed on unit mass of absorbent (g/kg)
• K_f is Freundlich sorption coefficient ([g/kg] [L/g]n)
• n is Freundlich exponent
• C_e is the equilibrium adsorbate concentration in the solution and (g/L)

Simplified form of Freundlich isotherm

Procedure of conducting the experiment of determining of adsorption isotherm of aqueous oxalic acid on activated charcoal

1. Prepare 500 ml of a standard oxalic acid (0.1 mol /dm-3) solution.
2. Clean, dry and label eight region bottles and weight accurately 1 g of charcoal into each region bottle.
3. Pipette 100.00, 80.00, 60.00, 40.00 and 20.00 mL of the oxalic solution into bottle 1-5. Add 0.00, 20.00, 40.00, 60.000 and 80.00 mL of distilled water to make a total volume of the total volume of 100.00 ml in each bottle.
4. In order to carry out a blank titration to correct the acidity of charcoal, add 100.00 mL of distilled water and 1 g of charcoal to 6th bottle.
5. Shake the bottle periodically and allow standing for above an hour to attain equilibrium. Filter each bottle, discard the first few milliliters (above 10 ml) and collect the rest of the filtrate in labeled flasks.
6. Pipette 5.00 ml of each of the solution from flasks and determine the oxalic acid concentration (C_e) by titrating with the standard NaOH (0.05 mol dm-3) using phenolphthalein as the indicator.
7. Carry out the determination of oxalic acid in the solution in the 8^th bottle after 1 hour and 30 minutes.

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Results and Calculations

1.0 Derive two equations for Q_e and C_e.

Using Langmuir isotherm,

This gives a y = mx + c equation which gives a linear relationship between Q_e and C_e.

Using Freundlich isotherm,

This gives y = mx + c equation which gives a linear relationship between log (Q_e) and log (C_e).

2.Calculate and tabulate the following quantities C₀, C_e, log(C₀), log (C_e), 1/C_e. Where C₀ is the initial concentration of oxalic acid, m is the mass of oxalic acid, V is the volume of NaOH required to neutralize oxalic acid in bottle 1-5, V₀ is the volume of NaOH requited neutralizing free acid in charcoal (bottle 6).

Notations

Reaction of oxalic acid and sodium hydroxide

H₂C₂O_4(aq) + 2NaOH_(aq) → Na₂C₂O_4(aq) + 2H₂O_(l)

Molecular mass of H₂C₂O₄ = 90 g mol^-1

Equations used for calculating consumed NaOH volume for titration with oxalic acid

Volume of NaOH consumed for oxalic acid= V_av - V_n

According to the stoichiometry:

Table: Calculate mass of adsorbate absorbed on unit mass of absorbent and equilibrium adsorbate concentration in the solution

Plot Langmuir (1/Q_e vs 1/C_e) and Frendlich {log(Q_e) vs log(C_e)} isotherms, and determine whether a Freundlich or Langmuir isotherm provides a better fit to the data.

Langmuir isotherm obtained from experimental data

Freundlich isotherm obtained from experimental data

Calculate the adsorption capacity of oxalic acid on activated carbon.

• log(Q_e) = log(K_f) + n log(C_e)
• C = log(K_f)
• C = 2.2416
• K_f = 174.4215

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Questions

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